1. Field of Invention
The present invention relates in general to free-space, reflective diffraction gratings, and more particularly to free-space, reflective diffraction gratings having a high throughput efficiency.
2. Discussion of Related Art
Diffractive elements are used in many application areas to spatially separate the component wavelengths of polychromatic light. For example, in telecommunications applications, diffractive elements may be used to spatially separate the channels of a wavelength division multiplexed (WDM) signal for subsequent processing (e.g., gain equalization or switching). Examples of diffractive elements for achieving spatial separation, include fiber Bragg gratings (FBG), and reflective and transmissive free-space diffraction gratings. Each of these diffractive elements offers performance advantages and disadvantages. A diffractive element's suitability for a specific application is dependent on characteristics such as throughput efficiency, dispersion, polarization-dependent loss (PDL), as well as the diffrative element's size, manufacturability, expense, and sensitivity to environmental conditions.
As is well known, reflective diffraction gratings are comprised of a reflective surface having a plurality of spatially-separated, reflective grooves and intervening ridges that induce a spatial, periodic phase delay on a wavefront of a beam of light. The phase delay causes diffraction of the beam of light and angularly separates the beam into its component wavelengths of light. Conventional reflective, free-space diffraction gratings offer unique characteristics that make them suitable for many applications. However, as discussed below, they have performance shortcomings. Reflective, free-space diffraction gratings are referred to herein below as diffraction gratings or simply as gratings.
The term “throughput efficiency” is defined herein to mean the logarithm of the ratio of the useable portion of the output optical power to the total optical power incident on the diffraction grating. In many applications, the useable portion of the output power corresponds to a single diffraction order. This diffraction order is referred to herein as the diffraction order of interest. A high throughput efficiency typically means greater than approximately 85%.
In telecommunications applications, higher throughput efficiency often translates to a reduced need for amplifiers to boost the signal. Further, in many systems, a signal is first demultiplexed by the grating and then re-multiplexed by the grating. In such systems, the desirability of gratings having high throughput efficiency is compounded by the fact that the effects of low throughput efficiency are doubled.
The term “Polarization Dependent Loss” (PDL) is defined herein to mean the logarithm of the ratio of the throughput efficiency of the TE-polarized portion light (expressed in power) to the throughput efficiency of the TM-polarized light (expressed in power). In many applications, unpolarized light is incident upon a grating. Further, in such systems it is typically desirable that the transmission of light be independent of polarization so as to avoid producing a polarized output (i.e., it desirable that the PDL be as near to zero as possible). However, conventional diffraction gratings typically transmit light in a highly polarization-dependent manner, particularly when they provide a high dispersion.
The term “angular dispersion” is defined herein to mean a variation of the wavelength of light as a function of angle. For example, processing of a polychromatic signal by a diffraction grating gives rise to a corresponding dispersion. Dispersion by free-space reflective diffraction gratings may be characterized using the well known grating equation,
                              sin          ⁡                      (                          θ              m                        )                          =                              sin            ⁡                          (                              θ                1                            )                                +                      m            ⁢                          λ              d                                                          (        1        )            where θi is the angle of the incident beam with respect to the grating normal, m indicates a diffractive order, θm is the angle of the mth diffractive order with respect to the grating normal, λ is the wavelength of the light, and d is the period of the grating.
More particularly, for a plurality of wavelengths incident on a diffraction grating at a single angle, angular dispersion is given by the equation,
                                          ⅆ                          θ              m                                            ⅆ            λ                          =                  m                                    d              ⁢                                                          ⁢              cos              ⁢                                                          ⁢                              θ                m                                      ⁢                                                                                    (        2        )            
The term “linear dispersion” is defined as the product of angular dispersion and a selected length. Devices that process the spatially-separated channels of a WDM signal typically require a specific linear dispersion. Accordingly, it is typically desirable to have angular dispersion be as large as possible to minimize the space necessary to achieve a given amount of linear separation. As used herein below, the term “dispersion” refers to angular dispersion.
Traditional free-space diffraction gratings (i.e., gratings having a plurality of grooves and ridges without the use of any of the enhancement techniques described below) may be selected to provide a suitable throughput efficiency, dispersion, and PDL for a particular application. However, in many instances, a single traditional grating is not capable of producing suitable amounts of each. For example, a grating having a grating frequency of 1800 lines/mm is known to have a 90% throughput efficiency for TM-polarized light in a wavelength range of 0.8 to 0.9 μm. However, the efficiency of the TE-polarized light in this wavelength range is below 50%.
Traditional free-space diffraction gratings having a 1800 lines/mm grating frequency also provide high throughput efficiency for the TE-polarized light in a very narrow band around λ=0.5 μm; however, the efficiency of the TM-polarization in this wavelength range is below 50% (see pg. 78 of Diffraction Gratings and Applications, by Loewen, published by Marcel and Dekker, 1997). Additionally, traditional free-space gratings may be selected to provide high efficiency for both TE-polarized and TM-polarized light in a selected wavelength range; however, the dispersion in the selected range is low.
One example of an enhancement technique that may be applied to a diffraction grating is the addition of a blaze angle to the ridges comprising of the grating. A blazed grating has ridges shaped to maximize the efficiency of the grating at the blaze angle. For example, in the range 1525 nm to 1570 nm (i.e., the C-band)), a gold-coated grating having a frequency of 600 lines/mm, a blaze angle of 28–34°, and which is aligned in a Littrow mounting arrangement (i.e., an arrangement in which light in the minus-first diffraction order traces back over the path of the incident light), provides a throughput efficiency in the range of 80 to 85%, a PDL below 0.3 dB, and a dispersion of 680 μrad/nm for the first diffraction order.
The dispersion of such a blazed grating is relatively low thus requiring a large package size to achieve adequate linear dispersion. For example, for a WDM channel spacing of 0.8 nm in wavelength (100 GHz in frequency), the path length necessary to achieve a linear dispersion of 100 μm between channels, is on the order of 360 mm (180 mm to a lens and a further 180 mm to the focus). While a grating may be selected which provides greater dispersion, such gratings also produce increased PDL.
Other modification techniques have been employed to provide increased dispersion. For example, the ridges and grooves may be configured such that the diffraction order of interest is a second (or higher) diffraction order, which provides an increase in dispersion relative to a grating for which the diffraction order of interest is the first diffraction order. Such gratings are referred to as echelle gratings. While echelle gratings provide increased dispersion, they are limited by their throughput efficiency. Yet another example of modified grating is a “grism” (a combination of grating and prism). Grisms are known to provide high throughput efficiency and dispersion, but for only a single polarization.
Grating systems which “work around” the limitations of traditional gratings have provided improved dispersion, throughput efficiency, and PDL. However, such systems have been complicated and expensive to produce. Examples of such systems include systems employing polarization diversity techniques, and systems using multiple dispersive elements.
Polarization diversity is a technique in which the TE-polarized light and the TM-polarized light are spatially separated and one of the polarizations is rotated 90-degrees, such that all of the light is either TE-polarized or all of the light is TM-polarized. Accordingly, systems based on polarization diversity provide an opportunity to avoid limitations arising from reduced throughput efficiency of either the TE-polarized or TM-polarized light.
One example of a system based on polarization diversity includes components to spatially separate the TE-polarized portion of light from the TM-polarized portion of light, and components that convert the TE-polarized light to TM- polarized light. Subsequently, all of the light (now TM-polarized) is processed by a traditional diffraction grating selected to have a high-efficiency and high-dispersion for TM-polarized light. Drawbacks to polarization diversity techniques include the added expense resulting from the increased number of optical components, increased system size arising from the increased number of components, and an increase in alignment demands resulting from the increase in the number of the optical components.
An alternative “work around” technique employs multiple dispersive elements (each element having a relatively low dispersion) to achieve a relatively large aggregate dispersion. For example, in some systems, multiple prisms or multiple gratings may be placed in series to increase dispersion. Typically, each such element is selected to have a low PDL. Similar to the polarization diversity-based systems, drawbacks to the use of multiple dispersive elements include added expense resulting from the increased number of optical components, increased system size arising from the increased number of components, and an increase in alignment demands resulting from the increase in the number of the optical components; in addition, the use of multiple dispersive elements also results in a substantial decrease in throughput efficiency.
While some of the above grating systems may provide appropriate throughput efficiency, dispersion or PDL dependent loss for some applications, there remains a need for grating elements providing appropriate combinations of each characteristic, as well as providing appropriate size, manufacturability, and expense.